Intensity-modulated radiation therapy (IMRT) is an advanced type of high-precision radiotherapy used to deliver precise radiation doses in medical procedures. IMRT modulates the intensity of multiple radiation fields originating from different directions to primarily focus on a region of the patient's body that generally conforms to the shape of a target volume, such as a malignant tumor, while exposing surrounding normal tissue to reduced levels of radiation. Typically, a detailed treatment plan is developed using computed tomography (CT) or magnetic resonance imaging (MRI) of the patient, along with computerized dose calculations to determine the dose intensity pattern.
In general, IMRT treatment plan optimization processes have been based on a cost function, which scores the achieved dose distribution. The cost function generally is defined such that more desirable treatment plans are associated with a microstate that results in relatively reduced cost function values. The microstate generally includes parameters needed to deliver the dose to the patient. The cost function typically is interactively defined, or specified, by a user, such as a medical technician or a physician, to develop the treatment plan with respect to certain machine parameters, optimal fluence, or the like.
In a typical optimization process, the user employs a set of treatment planning tools to specify the cost function contribution from various factors, such as requested target distributions, the dose level of organs-at-risk (OAR), dose distribution in normal tissue, or the like. In practice, the user generally specifies a set of optimization objectives, each of which is correlated with a term of the total cost function. Optimization objectives include, for example, dose-volume-histogram (DVH) objectives, normal tissue objectives (NTO), and so forth.
Existing treatment plan optimization methodologies can have drawbacks when used to develop radiation therapy plans. Defining the cost function to take into account clinical goals can involve a complex, often iterative, process requiring significant clinical experience. In general, the user has limited options with regard to plan improvement. Some plan details, for example, attaining a desired level of normalization, can be relatively difficult to control.
In other cases, the clinical goal definitions allow for multiple treatment plans, but it can be difficult to determine which of the plans is dosimetrically superior. In such cases, additional optional goals have been applied to the plan.
In addition, the dose distribution for multiple regions of normal tissue is controlled by a single NTO, which cannot guarantee achievement of clinical goals, and related parameters must be manually set by the user to comply with intended clinical goals. As a result, it often can be necessary to define virtual structures, that is, specified spatial regions having no direct anatomical function, in an attempt to control normal tissue exposures, for example, in the vicinity of target volumes or near the skin.